The murine src-containing retrovirus (MRSV) alters the proliferation and differentiation of hematopoietic cells in vitro. The ability of src to alter the proliferation and differentiation of pluripotent hematopoietic stem cells in long term bone marrow cultures is unique. The means by which this changes occurs is not understood. The target cell for MRSV, which causes the change in hematopoiesis in long term bone marrow cultures will be identified. Helper-free virus preparations of MRSV will be used to infect either the hematopoietic stem cells or the stromal cells in the long term bone marrow culture to determine which cell is responsible for altering hematopoiesis. Stem cell differentiation and proliferation will be measured by means of the in vitro CFU- C and CFU-mix colony assays and by the in vivo spleen colony assay (CFU-S). Once the cell responsible for altering hematopoiesis has been identified, the mechanism by which src causes this change will be investigated. The MRSV-infected long term bone marrow culture system is significant because no other virus has been shown to extend the proliferative capacity of hematopoietic stem cells without causing leukemia. A src virus encoding a second marker, that can be detected with fluorescent antibodies, will be used to infect mice. Virus-infected hematopoietic cells will be isolated from these animals with a fluorescent activated cell sorter and the properties of these cells characterized in CFUC, CFU mix and CFU-S colony assays. This will determine whether src alters the proliferation and differentiation of hematopoietic cells is altered in vivo as well as in vitro. The results of these studies may provide insight into the mechanism controlling the decision of a stem cell to proliferate (self-renewal) or differentiate. Such insights may prove to be clinically important in bone marrow transplants and in targeting chemotherapeutic drugs.